There are many in the market. I suggest you should draw your own spec. and make an evaluation. Some of the basic features one should look for :-
o Support kinds of voice coding;
o Support incoming call on show, restricting, and holding, disconnection, call transfer, call divert, DTMF, dialing project, etc;
o Support conference call;
o Support upgrading encrypts configuration files by TFTP or HTTP;
o Features like ATA(SIP2.0), route / bridge /DHCP, Ethernet Port, FXS and other should be evaluated before buying.
I purchased Broadcore Hosted VoIP PBX system and I am satisfied with the device. I had a bit of initial problem while installation but they attended and resolved it.